As an example of a production process of a propylene block copolymer having a good balance among its impact resistance, stiffness and workability, JP 3325419B discloses a production process comprising the steps of (1) homopolymerizing a predetermined amount of propylene or copolymerizing a predetermined amount of propylene with a predetermined amount of other olefin, thereby forming a slurry of the homopolymer or copolymer, (2) transferring the slurry to a reactor containing a combination of an ether or ester with a dialkylaluminum halide, and (3) further copolymerizing ethylene with propylene in the reactor.